The final installment of Blackadder, Back & Forth is set on New Year's Eve in 1999. The modern day Lord Edmund Blackadder and his idiotic servant Baldrick construct a time machine designed by Leonardo da Vinci as part of a prank on his party guests, betting them to give £10,000 each if he brings back a nominated item of historic significance for him to bring back - namely a Roman centurion helmet, the Duke of Wellington's iconic wellies, and a stinking pair of 18th Century underpants (provided by Baldrick). To their surprise, the time machine actually works, landing in the Cretaceous Period where they are attacked by a Tyrannosaurus Rex. Baldrick tosses the collected props at the dinosaur, successfully killing it with his underwear (apparently wiping out all dinosaurs with the stench).
Blackadder and Baldrick try to return home, but Baldrick never got around to writing in dates to know where they are going. By chance, the machine lands back in Blackadder's manor, but during Elizabeth I's reign. Blackadder manages to escape execution by giving the queen some Polo mints, and she gives him her crown as a prize. Heading back to the machine, Blackadder bumps into William Shakespeare. He gets Shakespeare to give him an autograph using a ball point pen, then punches the Bard, blaming him for misery his plays will bring to children in the future. Blackadder returns to the time machine, though forgets to take the pen back from Shakespeare.
After a brief encounter with The Future, Blackadder and Baldrick end up in Sherwood Forest and meet Robin Hood (based on Lord Flashheart), his merry men and Maid Marian. However, Blackadder questions the pointlessness of their good deeds, prompting the men to kill Robin by shooting him with arrows. Later, the time machine crash lands on The Duke of Wellington, preventing the Battle of Waterloo from being won by the British, though Blackadder steals the Duke's wellies before things kick off. The time machine lands in Roman Britain by Hadrian's Wall. Blackadder and Baldrick spot their Roman counterparts, who learn from General Melchecus that they are to return to Rome. A horde of Scottish warriors charge the wall, prompting Blackadder and Baldrick to flee in the time machine.
Baldrick comes up with a cunning plan to nearly drown in the machine's toilets to remember how the machine's levers were set up before taking off. Blackadder shoves Baldrick into the loos until he remembers. They return to the present only to discover history has been drastically changed — the French rule Britain, no one knows Robin Hood, and Shakespeare is only known as the inventor of the Biro. Blackadder and Baldrick make amends in the past, and Blackadder wins his bets. Melchett comments on how history could be changed by an unscrupulous person, giving Blackadder a plan, and he goes on a final trip in the time machine.
Blackadder's friends go upstairs to watch the countdown for the 21st Century on television. A royal car rolls up and out steps Blackadder - now King Edmund III - accompanied by Maid Marian as his queen. Baldrick has become Prime Minister, and maintains a strong friendship with Blackadder. The end credits end with a teaser claiming the sequel will be released in the year 3000!
- Aside Glance: The final shot has Edmund and Baldrick smirking at the audience at their new position.
- Book-Ends: The first Blackadder series was all about Prince Edmund wanting to become king. In this special, the last of the Blackadder series, his descendant finally achieves it.note
- Bungling Inventor: Good news—Baldrick's time machine works. Bad news—He used a slot machine for the year readout and forgot to put numbers over the fruit symbols.
- Canis Latinicus: Melchett's Roman ancestor speaks perfect Latin, but ends his report with "Beeeh-us!"
- Expy: By the admission of both Rowan Atkinson and the writers, the present-day Lord Blackadder seen here is effectively the Blackadder II incarnation transplanted to the 20th/21st century. Also, his time machine is more or less one for the TARDIS.
- Good Is Dumb: How Blackadder convinces the Merry Men to turn on Robin Hood. However, he quickly realises what a big mistake that was.
- Grand Finale: Unless there really is another Blackadder entry planned for release in the year 3000.
- Identical Grandson: Everyone! In fact, the only characters who don't have an identical ancestor or descendant somewhere else in the Blackadder series are William Shakespeare and Maid Marian.
- Lighter and Softer: Though it still has the series' trademark wit and Black Comedy, the vulgarity is toned down noticeably. Blackadder, while still a Jerkass, is more a petty schemer that gets caught in a plan gone wrong, and perhaps most noticeably the film ends on a surprisingly happy note that isn't at the expense of any of the main characters.
- My God, What Have I Done?: Blackadder's reaction when he discovers that he's altered history and caused Britain to become a French colony. It's implied that he could have lived with Robin Hood and the works of Shakespeare being erased from history, but he ends up restoring everything. For a few minutes, at least.
- Naked Apron: How Baldrick serves dinner at the beginning of the episode. Definitely a Fan Disservice example.
- Shout-Out: The time machine falls on the Duke of Wellington in a similar manner to Dorothy's house in The Wizard of Oz.
- Throw the Dog a Bone: The Blackadder line (and even the Baldrick line) finally coming into greatness, with Edmund king and Baldrick prime minister.